Voltage monitoring circuitry is applied to respond to power source irregularity, such as power crash or power fault, for prevention of a load circuit connected to the power source from malfunctions. For a simple illustration, FIG. 1 shows a power supply 10 providing a voltage VDD to a processor 14. If any fault is occurred in the power supply 10, the voltage on its output 12 may decrease down to smaller than its rating and, as a result, to cause error outputs 16 from the processor 14 or damages to the processor 14. For the sake of prevention, a detector 18 is added to monitor the output 12 of the power supply 10 and sends an alarm signal 20 to the processor 14 if any power drops over its threshold is happened. To generate the alarm signal 20, the detector 18 compares the monitored voltage 12 with a predetermined threshold. However, a constant threshold is not adaptive to various applications, and thus one or more detector chips are needed to replace the original one once the application condition is changed. To meet such requirement, the detector chip manufacturers have to design and manufacture chips for each spec, and the chip agents and system assemblers also have to prepare various chips.
For adaptive detector to monitor the supply voltage, various threshold voltages are provided for the detector 18, as shown in FIG. 2. With this improved circuit, a comparator 22 compares the supply voltage VDD from an input 24 with the other input 26 that receives a varied reference voltage Vref from a threshold generator 28 to generate a monitoring signal 32. To generate a threshold voltage from one of a plurality of predetermined levels, one or more select signals 30 are provided to program the threshold generator 28. Typically, a voltage divider is included in the threshold generator 28 to define the threshold levels. As is well known, a binary signal determines two states for the threshold level. When the level number increases, the pin count of the detector chip 18 becomes more in doubled. For example, three programming pins can determine eight (=23) levels for the threshold voltages. In addition to the increased cost resulted from the more pins, the resistors of the voltage divider occupies huge chip area and consumes high power when large number of threshold voltages are desired. These resistors further introduce much more noise to the circuit. It is therefore desired a programmable voltage supervisory circuit and method with minimum programming pins and low quiescent current to monitor a power source.